Limp Mode Dangerous?

[Sulaco]

I did a search and went through the first 30 or 40 threads that popped up and found lots about limp mode, but nothing on what I needed:

My car is kicking itself into limp mode roughly every single time I have to hit the gas pedal now. I'm pretty sure I just need to bite the bullet and get the wiring harness (no CEL, new plugs).

But my question is this. I HAVE to drive the car sometimes. Especially when I go home for thanksgiving (3 hour drive). Is limp mode dangerous? Does it actually save gas (I lose 2 cylinders when she's in limp mode!)? Could the car catch fire or anything? When it kicks into limp mode, I just restart the car immediately--I don't drive around in limp mode. But if I DID drive around in limp mode, would it be very bad for the car, or would it actually prevent any further damage?

[audiophile]

i think my car does the same thing, whenever the kickdown switch is hit.
i thought it was the MAF, but i replaced it and nothing:confused:

[alphanumeric]

Originally posted by audiophile
i think my car does the same thing, whenever the kickdown switch is hit.
i thought it was the MAF, but i replaced it and nothing:confused:

I heard something about having to take it to the dealer to fix your problem.. even tho u replaced your maf ... its just another way for them to take your money

[audiophile]

i had heard that you would have to reset the fuel maps (or something ) to get the ECU working properly:confused:

[Sulaco]

Help?

[benzaddict]

Yes, "limp mode" is very dangerous, just ask your girlfriend!! j/k...

Seriously, there are several variations of "limp mode" depending on what system/components are failing. for example the transmission can go into "limp mode" when there is a transmission fault and the result is it sticks in whatever gear it failed in until you come to a stop, then it sticks in second gear for the duration of the trip. There is also a " limp mode" of your throttle acuator, when it fails it raises the idle to about 1200rpm and flucuates slightly and won't respond to any throttle inputs.

What your minimal description sounds like to me is "cylinder shut-down mode", where the fuel injection computer is detecting a misfire from a particular cylinder and shuts down the cylinder by not energizing the injector or ignition coil, and protecting the catalyst from damage due to passing unburned fuel through it. What is very common on your model is that the wiring harness breaks down inside the valve cover going to the coils and as soon as the coil wires short, it cuts both cylinder's related to that coil (1-6, 2-5, 3-4), which is "cylinder shut down mode". As soon as you cycle the key and restart it "refreshes" until the next misfire detection, which seems to be happening to you. While your engine is in "cylinder shut down mode" it is not necesarilly damaging anything, but it's not doing it any good either. It basically turns those 2 cylinders into air pumps until you cycle the key again.

So if you're asking whether or not you could damage anything by driving it like that, my answer you be most likely "no" as long as it is only for short trips, but if you're planning over a 3 hour trip I would think you're crazy. Consider the fact that your cylinder walls/piston rings need to be lubricated, and part of that lubrication is from the fuel mixture that is sprayed into the cylinder and ignited. If 2 of your cylinders are in "shut down mode" they'll be getting minimal lubrication. Sure there is lubrication from underneath the piston, but for a 3 hour trip I could see pretty good damage resulting.

Do yourself a favor and replace the harness, even of you have to borrow the money. You'll be even more upset at yourself if you have to buy a new engine instead of just a harness. Just my opinion..... hope it helps.

[Sulaco]

That is precisely the kind of help I was looking for. Thank you VERY much, benzaddict.

In other threads I have thoroughly discussed and described the symptoms my car is having, and I'm convinced it's a problem with the harness. It's most certainly losing spark to one of the coil packs and going into cylinder shut off mode. But it's getting worse every day. At first it only did it if I half-throttled (but when I feathered it or floored it nothing would go wrong). Now, no matter how I accelerate it goes into limp mode. But if I cruise, it stays in normal mode. Most of the time if I'm slowing down, I can manually gear it down to keep the RPM's up a little and keep the fuel/fire going, and then I can gently acccelerate again and it will be ok. But if I come to a stop, I have to hold the brake and hold the gas pedal a little bit (once again, to keep the fuel/fire going a little higher) otherwise it will kick into limp mode while just sitting stopped.

I know I need the harness, and yes, if I get it now I WILL have to borrow the money. But already being $24,000 in debt for school is bad enough as it is. But my birthday is this weekend, maybe I can just collect some spare cash here and there and save it till christmas and collect some more. Who knows!

Once again, thanks for the VERY good reply.

[c280nz]

theres a good write up and picture tutorial on changing the w/h yourself. if you purchase one off an aftermarket company/ebay and then install yourself you will save serious $$$ prolly cost you less than half.
good luck either way

[Sulaco]

Yeah, I've found them at mercedesshop.com for $569, and that's the cheapest available. I know I can replace it myself, too.

[Abuimad]

well hope u replace them soon, i love ur car and its color, thing is clean so good luck on replacing the wiring harness, and hope to have that car bounce back better than ever